Proper calibration of tools, implants and other components used in computer assisted surgery (CAS) procedures is vital.
Several CAS procedures require the determination of the center of rotation of a circular object, a cup or a half-sphere. For example, total hip replacement (THR) procedures require that the acetabular cup implant be properly calibrated such that the CAS system accurately knows the exact location in space of the center of the cup, which is typically held by an impactor tool having a handle defining an axis through which the center of rotation of the cup is concentric.
Several techniques are currently employed to determine the center of prosthetic cups and other hemispherical shaped objects used in CAS procedures. One common technique involves immobilizing the cup in question in a reference block and subsequently employing a digitizing CAS pointer to determine individually and sequentially at least three points along the outer circumference of the cup's rim. The CAS system then calculates the corresponding center of the circular cup based on three known points acquired on its circumference. While generally effective, several disadvantages exist with this procedure and the associated equipment required. For example, the need for a relatively large reference block capable of immobilizing the cup and the time required to digitize the individual points on the cup are both disadvantages for which improvement is sought. Further, the associated handling error which can result from digitizing points on the object retained within the reference block can cause differences between the calculated center determined by the CAS system and the true center of the circumference. These differences, while sometimes small, can be significant enough to considerably affect the results.